A gas chromatograph (GC) is a powerful instrument for analyzing chemicals, particularly organic chemicals. This instrument, when coupled with a mass spectrometer or other specific detectors, is considered the analytical instrument of choice in many laboratories around the world.
All advanced GC systems currently commercially available are equipped with capillary columns. Over the last 30 years, addition of the capillary column has increased the power of this instrument. Previously, all GC were equipped with packed columns. These columns have inside diameter as large as 4 mm, and are only a few meters long, for example about ten meters long. To improve the separation power of the GC, instrument makers replaced the packed columns with capillary columns. These columns are much smaller in diameter than the original packed columns, for example with an inside diameter of about 0.3 mm, but may be as long as one-hundred meters.
With this capillary type of column, chemists have the power to separate closely related compounds, thus, greatly increasing their ability to identify and quantify chemicals. However, the capillary column has one significant drawback in that it is unable to accept large sample volume.
By way of illustration, during the introduction (injection) of a sample into a conventional GC via an injection port lined by an inlet sleeve, the sample is vaporized in the injection port, which greatly increases the volume of the sample. For the solvent hexane, for example, a typical injection of 1 microliter (.mu.l) will have an expanded volume of about 200.0 .mu.l. A 5.0 .mu.l injection will have an expansion volume of almost 1000 .mu.l
In a conventional GC, the largest inlet sleeve lining an injection port has a maximum volume of about 942 microliters. As all expanded sample gases have to enter the capillary column, the capillary column is flooded if a sample volume greater than 5.0 .mu.l is injected. The large expansion volume also leads to an increase in the pressure inside the injection port. This can lead to problems such as backflash and sleeve breakage.
The above-discussed drawback thus limits to few microliters the sample size which can be usefully injected into a conventional GC equipped with capillary column. Several GC manufacturers offer optional replacement injection systems to allow somewhat larger sample injections. Such replacement systems may increase the sample size capability only by about 250 .mu.l and the original injection system must be removed to accommodate the replacement. Accordingly, the cost of the optional equipment and the labor cost of replacement may be sufficiently high that such a replacement does not provide a cost effective way to improve an existing system.
There is a need for an "add-on" device that can be attached to an existing injecting system of a conventional GC, and allows for larger sample injection. Preferably such a device would allow the injection volume of a conventional capillary-column GC to be increased from a few microliters to over one milliliter. Increasing the injection volume to over one milliliter could improve the detection power of a conventional GC over 1000 times (depending on how large a volume is injected).